Concrete is a durable construction material. Reinforced concrete (often including rebar) is competitive with other durable building technologies, like steel frame or traditional bricks-and-mortar. But a building's strength and stability is also a function f how a building's framing works in conjunction with its foundation and other building elements to provide strength and stability for the structure. As recognized by FEMA, the Federal Emergency Management Agency (“Building Framing Systems and Best Practices,” available at: https://www.fema.gov/media-library-data/20130726-1708-25045-9326/chapter7.pdf (2013)), properly designed and constructed building framing is important in all locations; however, particularly in coastal areas where wind, flood, and other loads can be extreme due to significant weather events, ensuring proper building framing is critical. Framing must adequately transfer all gravity, uplift, and lateral loads to the foundation.
In buildings (including residential structures), framing systems typically consist of the roof structure that supports the roof deck, exterior and interior load-bearing walls, beams, girders, posts, and floor framing, if any. Shear walls provide strength to resist lateral loads.
Exterior walls are one of the most important elements within the framing's load path from the roof to the foundation. Exterior walls must resist loads imposed on them (particularly by wind or seismic activity) and typically must function as assemblies to provide stability for the entire structure.
Three types of loads can be imposed on exterior walls: (1) loads that exist out of the plane of, or perpendicular to, the wall are imposed primarily from wind but can also result from seismic activity. All exterior walls are exposed to these out-of-plane forces; (2) vertical loads also called axial loads) are transferred into some walls from the roof (or upper-story walls) above. The vertical loads can be downward-acting gravity loads that result from the weight of the structure or upward-acting (uplift) loads from wind or seismic events. Uplift and gravity loads are considered in-plane loads since they occur within the plane of the wall, but act along the vertical axis of the wall. All load-bearing walls are exposed to in-plane gravity loads (such as the dead loads of non-load-bearing walls). In addition to the in-plane gravity loads, many walls are also exposed to uplift loads; and (3) in-plane horizontal loads can also exist in some walls, which typically result from wind forces imposed on building surfaces that are perpendicular to the walls. For example, wind loads acting on a building's roof and front wall create horizontal loads in its left and right walls. Those horizontal loads are collected through horizontal diaphragms such as floors and roof deck assemblies, and are called “shear loads.” The walls that are needed to resist these loads are called “shear walls” or “shear panels.” Shear walls that function as load-bearing walls are exposed to all three types of loading. Shear walls also provide lateral stability for a structure.
By way of background, Section 8602.10 of the International Residential Code (IRC) provides prescriptive construction details and requirements for braced wall panels for buildings exposed to 3-second gust basic wind speeds less than 110 mph (less than 100 mph in hurricane-prone areas). The Wood Frame Construction Manual (WFCM) provides prescriptive shear wall details for 3-second gust wind speeds from 85 mph to 150 mph. In addition, Section 305.4 of the SSTD-10 Standard for Hurricane Resistant Residential Construction provides shear wall designs appropriate for use in buildings exposed to wind speeds up to 110 mph (fastest mile). Shear walls may also be constructed with masonry, concrete, insulated concrete forms (ICFs) and with structural insulated panels (SIPs). SIPs consist of wood structural panels which sandwich a rigid insulation core, which is typically polystyrene or urethane.
When analyzing shear walls, two classifications of shear walls exist. Segmented shear walls are full-height, fully-sheathed wall segments that function independently to resist lateral loads. Perforated shear walls contain framed openings for windows and doors. Perforated shear walls rely upon continuous structural elements over windows and door openings to make the shear wall function as a single unit. Generally, greater lengths of perforated shear walls are needed to resist lateral loads than segmented shear walls. Also, in perforated shear walls, more attention in the detailing and design is needed above doors and windows, where framing functions as drag struts. The greater attention is needed to ensure that the drag struts and their connections are adequate to transfer in-plane loads through the shear wall.
The integrity of the overall building depends not only upon the strength of shear walls and other building components, but also on the adequacy of the connections that exist between them. Critical connections occur throughout the structure, but, in most houses, the most critical connections exist where the roof system connects to supporting walls; at openings (e.g., for windows and doors) and headers in the walls; where walls connect to each other at floor levels; and where walls connect to the foundation.
Shear walls (whether segmented or perforated) must be anchored to the foundation (or the shear wall below when on an elevated floor) to complete the continuous load path within this area of the building. A proper anchorage or connection prevents the shear walls and, in turn, the rest of the structure from laterally racking, displacing, or overturning during a high-wind or seismic event. The reactions (loads) at the ends of shear walls are proportional to wall height. Taller walls develop larger reactions and require stronger anchors, and anchorage requirements for even small homes can be thousands of pounds. With larger shear forces, shear forces at tie-downs become greater and adequate tie-down and anchorage become more difficult to achieve.
When wind forces act on a building, the building must transfer induced loads. This requires connections to transfer the loads into shear walls through both compression (pushing) and tension (pulling). It is important that all of the elements of the building work together in order to create the maximum amount of structural strength and allow the building to maintain its shape and not compromise the building envelope. A failure in the connection or v of the members could result in structural failure.
The use of walls constructed from reinforced concrete is becoming more prevalent in communities impacted by hurricanes. When properly designed and constructed, these styles of walls can perform well when exposed to high winds. Typically, concrete construction is used in conjunction with wood-framed roofs and, in the case of multi-story buildings, wood-framed floors. But typical concrete and masonry walls lack the thermal performance required by the IRC and often require framed walls or thick furring to allow the addition of sufficient insulation.
Insulated concrete form wall sections can achieve improved thermal and structural performance with a single, reinforced concrete-wall sections because its permanent insulating form remains in place. One construction technique uses pre-formed panels to form a pre-insulated wall system that relies upon concrete for its structural integrity. Requirements for insulated concrete form construction are provided in IRC Section R611. The prescriptive designs for ICF are well-developed and detailed. Examples of prescriptive designs for concrete and masonry walls in high-wind regions are contained in Section 205.5 of ICC 600. Prescriptive ICF designs that meet high-wind requirements of the IRC may be found in Section 209 of ICC 600. But these specifications only contemplate basic wind speeds of up to 150 mph. There remains a need for insulated concrete form walls and related structural elements that meet even higher high-wind requirements, at a minimum of 200 mph, to withstand stronger hurricane and tornado forces.
In addition to achieving stability in the face of significant wind loads, there is a need for building elements that also exhibit superior energy efficiency and fire resistance. They also must permit speedy and low-labor-cost construction projects, particularly by conventional build crews.
Walls can be fabricated several ways known to those of skill in the art including: (1) fluid (usually with ready-mixed concrete) placed into concrete forms usually with the aid of concrete pumps; and (2) prefabricated (precast), on-site or in a factory, generally in a flat position and lifted with a crane into final position on the house foundation. These wall panels can be made on the ground adjacent to the house, or they may be made in a factory and transported (e.g., by truck) to the work/building site.
One method of lowering overall construction cost is to prefabricate construction elements (such as walls) rather than manufacturing them on the construction site. Several prefabricated building construction techniques are known in the art. Prefabricated construction techniques may involve creating concrete building elements including foundations, walls, deck assemblies, and roof assemblies. The elements may be manufactured to contain interior and external finishes, windows, doors, and utility distribution systems. These elements may also be assembled on a construction site into structures such as homes, commercial and other office buildings, noise-reduction walls, military installations, etc.
Some prefabricated construction techniques involve constructing portions of a building at a manufacturing facility and shipping the portions to a construction site. One problem with such techniques is the need to create structures of sufficient light weight to enable cost-effective shipping. Many light-weight prefabricated structures lack sufficient load bearing capabilities for certain applications, including the significant wind events discussed above. A need exists for structures having high structural integrity, durability, and low shipping and assembly costs.
Cast-in-place concrete walls can be formed with steel, wood or insulating foam boards. The advantage of using ICFs is that the finished product accomplishes both a structural and energy conservation function simultaneously. Walls must be structurally designed conventionally reinforced concrete shear walls in accordance with practice outlined in the American Concrete Institute document Building Code Requirements 318 and the International Building Code (IBC).
Several methods of fabricating ICFs are known in the art. Typically, the wall has been prefabricated in a flat position with the insulation boards firmly attached, either next to the building and tilted up with a crane to a vertical position, or assembled in an offsite plant, transported to the site and erected with a crane. One major disadvantage to this fabrication method is the cost. Shipping, in particular, is expensive and time consuming. Another disadvantage is the use of polystyrene forms that result in exterior foam insulation, which may provide a route for insects and groundwater to enter the walls. Accordingly, there remains a need for structures that are designed to achieve the benefits of elements such as ICFs (e.g., energy efficiency, moisture resistance, pest resistance) without the significant cost, and the ability to withstand significant forces or penetration events.